CHAPTER 1. INTRODUCTION
1.3. Thesis plan
In chapter 2, a literature review of the ACH system is done. Research on solar collector technologies is first presented. Secondly, heat driven machines development history, their performance status are explained. Thirdly sensible storage systems including water tanks and borehole storage are reviewed. Lastly, ACH systems using
16 more or less solar heater, absorption chiller in addition to the storage system are presented.
In chapter 3 and 4, deep explanations of the science behind each component used in the study systems are worked out. This includes the working principle of the components as well as their mathematicals models. The types of solar thermal collectors are discussed in details, showing the advantages and drawbacks of each technology. Absorption chiller and cooling tower are presented technically and the conditions most suitable for their use are discussed. Water storage tank along with borehole energy storage system is explained before case studies of systems proposed in the thesis are introduced.
In chapter 5, the methodology employed to obtain the study results is developed. First, the simulation tools are briefly introduced. Next, the meteorological data sources are compared to highlight the importance of getting good data in order to obtain accurate results especially if a commercial project is to be implemented. Lastly, the simulation flow of each case study is explained.
In chapter 6, the results of the three cases studies are presented and discussed. The merits of all systems are compared to each other in order to select the best system worth considering for an economic assessment. The economic parameters of interests are first explained, and then the calculations assumptions are given. The financial figures of the reference system versus the best proposed system are presented and discussed to conclude the chapter.
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CHAPTER 2
LITERATURE REVIEW
The literature review presents the research findings about the main components used to build the cooling/heating systems. They are: the solar collector, the ACH, and storage systems including hot/cold water storage tanks and BTES. Entire solar cooling systems are also reviewed to serve as a reference for the proposed study.
2.1. Solar Thermal Energy
The use of solar heat for processes such as cooking, drying, space heating cannot be traced back with certainty. Hundreds of years BC in Africa, ancient black Egyptians were using solar heat for space heating at night. Buildings were constructed such that heat is stored in the wall on daytime and released at night in the living space. In the 15th century, water distillation and agriculture product drying were done using solar energy [16]. In modern time, in 1866, Augustin Mouchot developed the solar parabolic collector showed in figure 2.1 [17]. Sunray was concentrated on a metal tube or a boiler that contained water. Steam was then produced to drive a steam engine that in turn powered various applications such as water pumps, printing presses and ice making machine. There are mainly two types of solar collectors: Concentrating collectors and Non-Concentrating collectors. Each type uses sun energy differently. In the first one, only the sun ray falling normally to the collector aperture area or the Direct Normal Irradiation (DNI) can be converted to heat. This means that a tracking system is necessary to ensure DNI is always received normal to the aperture area. In the latest one, sun ray coming from all directions or the sum of DNI, diffuse radiation (DIF) and reflected radiation (REF) can be converted to heat which means that a tracker is not necessary for these collectors.
18 Figure 2.1. Parabolic Solar Concentrator by A. Mouchot [17]
For non-concentrated collectors such as flat plate collectors (FPC) are used for domestic hot water production. A study by Ming et al. [18] showed that when the diffuse radiation is varied from 20% to 50% of the total radiation, the collector peak efficiency decreased by less than 1.83%. However, the corresponding heat loss decreased up to 7.83%. The DNI which has more energy tends to heat at a faster rate the collector, increasing heat loss considerably as the receiver area is large. To overcome this issue, some researchers attempted to reduce the receiver area by using a “V” shape reflector to reflect sunlight onto a much smaller area at the bottom of the structure.
Doing this, Ruhul et al. [19] obtained an efficiency increase by 10%. Collector Efficiency is also affected by the flow rate. At higher flow rate, the temperature difference between the inlet/outlet of the collect is reduced, which decrease the heat loss to the ambient [18,20,21]. However, higher flow rate increases pumping power which may cancel the efficiency improvement benefit.
Concentrating collectors such as parabolic trough collector (PTC) is more suitable where the DNI ratio is more important. When DNI to global radiation ratio is less than 60% the benefit of concentrating collector become less evident [22]. The main advantage of this technology is the possible high concentration ratio up to 70, allowing output temperature up to 500°C [23]. As mentioned for the non-concentrating collectors,
19 the efficiency of concentrating collectors also correlates positively with the flow rate. In fact, in addition, to lower the heat loss, higher flow rate also enhances the heat transfer coefficient through more turbulent flow. To promote turbulent flow, some researchers went further to introduce fins in the receiver tube [24]. Also, nanofluids with tailored characteristics have been used experimentally to enhance heat transfer in the receiver.
The technique achieved a collector efficiency boost up to 12.5% [25,26]. The heat loss for concentrating collectors is much less than that of the non-concentrating collectors as the receiver area is drastically reduced to a circular spot or to a partial area of a small diameter tube. Among the concentrating collectors, PTC is the most common one. Its peak optical efficiency can reach up to 63% while the theoretical limit is 75% [27].
2.2. Solar Thermal Absorption Cooling Systems
bromide chiller, improving its performance up of 50%. This was an important efficiency boost by considering that absorption diffusion refrigerator had a COP of 0.20-0.40.Today, commercial chiller has a COP typically in the range of 0.5-0.7. Experimental studies using a well-engineered fluids mixture, however, claimed a COP up to 0.8. The COP of the machine is governed by the three fluid streams temperature (Generator, evaporator, absorber condenser).
A chiller model for solar application has been validated experimentally by Marc et al.
[28] revealed that as the generator temperature is varied between 70°C and 90°C, the COP changed from 0.58 to 0.74. Also, the cooling capacity increased from 10kw to 30 kW. A similar conclusion was confirmed by several studies of ACH [29–32].
In an effort to improve the economic feasibility of ACH, a new approach consisting of combining multiple ACH units, by using the heat rejected by the first unit to power the second unit, then use the second unit rejected heat to power the third unit and so on has emerged. Each unit is termed as an effect. In practice, up to three effects chillers are
20 available commercially. Double effect chiller COP is often in the range of 1.2 to 1.4 [33,34]. Amari et al. [35] showed that direct fired double-effect chiller was much more economical compared to single effect hot water driven chiller. The payback of double effect chiller was found to be as short as 3.5 years.
Roland et al. [36] studied a compound parabolic collector with a 23 kW double-effect chiller driven at 160°C-200°C to found the overall COP of the system. It was 0.99 while the efficiency of the solar unit was 36.7%. By considering the chiller and the collector efficiencies, it can be deduced conservatively that the COP of the chiller was 2.4.
A solar-powered double effect ACH for subzero cooling temperature was studied by Catalina et al. [37]. They found a COP ranging from 0.58 up to 0.95 as sun power was changing. This low value was due to the subzero cooling which was too low compared to the conventional 6°C-12°C chilled water temperature used for building cooling application.
To cope with sun intermittency, dual power gas-solar chiller has been studied in Indonesia by Lubis et al. [38]. By varying cooling water temperature(28°C-34°C) and hot water temperature (70°C-90°C), a COP variation from 1.4 up to 3.3 was observed. A similar study was done in China for a hotel cooling/heating [39]. The system was working on single effect mode on the sun and in double effect mode on gas.
2.3. Ground Source Cooling/Heating
Geothermal energy is a solar energy stored in the earth because of the daily radiation the earth receives. There are two types of geothermal systems: low grade geothermal that uses the earth at a temperature typically below 50°C and high grade geothermal that uses hot water or steam out of the earth at 100°C up to few hundred Celsius. For each point on the earth, the annual average temperature is nearly constant.
As the point goes deeper from the surface, temperature approaches a constant value throughout the year. In the 17th century, this was proven by French Physicist Lavoisier who measured a constant soil temperature at 85feet depth at Paris observatory center [40]. In 1799, Alexander Von Humboldt noted that the changes in the measurement taken since 1680 were no more than 1 degree. In 1838 temperature measurement at the
21 Royal Edinburg observatory showed that the temperature variation at 25feet depth was 1/20 of the surface variation while at 50 feet the variation reduces to 1/400. The constant earth temperature motivated researchers to use the earth as a heat source/sink in building cooling/heating systems.
In Japan, a ground source heat pump using a horizontal ground heat exchanger loop buried at 0.5m depth and an electrical AC unit showed that the annual COP of the system was 3.26 [41]. To maximize the capacity factor of the system, it was sized at 60-70% of the heating load. In a similar study in Turkey, it was found that the COP of the heat pump was between 4.03 and 4.18 [42]. An economic analysis showed the system was more cost effective compared to others heating systems powered by natural gas, LPG or fuel oil. In a combined borehole thermal energy storage and solar collector study in Shanghai for building cooling/heating application, it was found that the COP of the system was 4.5 if the collector was used, and 4.2 if the collector was not used [43].
Solar energy utilization reduced electricity usage by 26.1%
A seasonal thermal energy storage study using borehole at 500m to 1500m deep and solar collector at 110°C was carried in Germany in 2015 [44]. The efficiency of the system was quite low at 15% for the first 3 years, then went up to 25% within 30 years.
In a 5 years simulation of BTES coupled to a solar collector in Finland, Rehman et al.[45] reported a heat storage efficiency improvement from 23% to 31% by using a control strategy to optimize the operation of the system. In a review of solar BTES by Gao et al.[46], the heat storage efficiency appeared to be between 9% to 41% over 5 years. The low efficiency of the system is due to the massive thermal mass of the earth which can dissipate heat easily and “swallow” any artificially introduced heat to the combination of solar collector-BTES-heat pump for building heating application, the system was able to save 32% of the electricity through an increased COP [48]. In this study waste heat rather than valuable heat will be the BTES heat source.
22 Night Cooling: Night cooling has already been investigated by numerous researchers.
Miguel et al. [49] showed in their study of office building night cooling, that a good slab design for night ventilation is an efficient solution to lower indoor daytime temperature. In a similar study, Lin et al. [50] found that night roof ventilation can reduce 10-24% of the AC energy consumption while reducing the indoor peak temperature by 3-5°C. Ali et al. [51] in an experimental study of tank water night cooling by natural convection and sky radiation found that water temperature can be lowered by 6.6°C during the night. However, this result is obtained by keeping the deep of the water at 0.5m, meaning a large area may be necessary to achieve commercial or utility scale cooling. Also, water consumption was as high as 4.5%. To avoid the use of scarce water for cooling, Ana et al. [52] studied night sky radiative cooling for a CSP plant and found that 80-90% of the cooling load can be satisfied with this technique. In these studies cooling is shifted from day to night without exploring the opportunities to cool water 24h/day at a continuous low flow rate which can increase cooling performance.
Chilled Water Storage: The performance of solar assisted absorption chiller with chilled water storage has been studied by Rosiek et al.[53,54]. A portion of the load was sent to the low cooling demand period, in order to have a less frequent ON/OFF of the chiller. The element of interest was the electric energy consumption which was reduced by 20% thanks to the new design. Similar study focusing on saving on peak power tariff has been done by Lin et al. [55] in 2014. A 40% power cost saving was achieved.
However, a reduction of the required chiller cooling capacity has not been explored by the previous study.
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CHAPTER 3
SOLAR THERMAL ENERGY
In this chapter, the fundamentals of solar energy technology is explained. It starts with the generalities of solar energy, explaining how solar power is generated, how it reaches the earth and which portion of the energy can be used for concentrated solar technologies. The existing types of solar thermal collectors are presented in more details as well.
3.1. Solar Energy Fundamentals
It is commonly accepted that the sun is the source of all forms of energy on earth.
In fact, in fossil fuel generation process, solar energy is first converted to carbon by plants through photosynthesis. Plants then constitute the green biomass or oil which is biomass deposited deep in the ground millions of years ago. For wind energy, the sun is at the center of wind flow by heating unevenly the earth, creating pressure differences which induce wind. The rain cycle necessary for hydropower is also powered by the sun through the evaporation process. In modern days, solar energy use as a renewable energy source is made in two ways:
(1) Solar photovoltaic which convert sunlight directly into electricity.
(2) Solar thermal which convert sunlight to heat; heat is then used for various applications including electricity generation, industrial heat process, space heating/cooling. Solar thermal cooling will be the focus of the thesis since air condition accounts for more than 50% of building energy consumption as mentioned in the introduction.
3.1.1. Generality of Solar System
The solar system is a structure born from the explosion of a supernova or star 4.5 billion years ago. It is formed by eight planets and one star, the sun, bound through gravity forces. Bodies such as moons, asteroids, and comets are found in the
inter-24 planets spaces. The system spans an estimated radius of 90 Astronomical Unit (1AU = 150Million kilometers, or sun-earth distance) centered at the sun. However, the farthest planet lies within 30AU away from the sun, the remaining space being covered by less known bodies. From the Sun in the outward direction, the planets are Mercury Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune as shown in figure 3.1. The first four planets are silicate planets. They have rocky surfaces, are composed of various solid materials including carbon, iron, calcium, potassium, and others silicate compounds.
Each planet has an atmosphere made of gases such as oxygen, carbon dioxide, nitrogen, water vapor, krypton, xenon, but differs in components composition ratio and gas layer thickness. Jupiter and Saturn are gas-giant planets, meaning they are made of relatively hotter gases wrapped around a small solid core. Uranus and Neptune are ice-giant, implying they are formed by very low temperature frozen materials. Unlike the planets, the sun is made almost entirely of helium and hydrogen. The key difference between the sun and the planets is that the sun is burning its material in a nuclear fusion reaction at its core, radiating the generated energy outward while planets are mainly energy receiver masses. The closer to the sun a planet is the more energy it receives from the sun. However, the atmosphere of a planet and the magnetic field of its core may act as a radiation shield or a greenhouse, making the effectively trapped sun energy less proportional to the distance sun-planet.
3.1.2. Sun-Earth Energy Transfer
The sun is a star with a diameter of , a mass of that lies at 150 million km from earth as shown in figure 3.2 Energy is generated in the core of the sun by nuclear fusion. In the process, two hydrogen molecules combine to form a helium molecule ( ). Since the mass of the reactant is less than the mass of the product, the missing mass is transformed into energy " "E following the Einstein formula:
(3.1) where is the missing mass to be converted to energy, the speed of the light the energy released.
25 Figure 3.1. Solar System[56].
The full equation of the thermo-nuclear reaction is thus ). Proton in the hydrogen molecule having a mass of 1.00794u while helium has a mass of 4.03176u, the mass themass difference converted into kilogram is ( ). Taking the speed of light as the released energy per reaction obtained with equation 3.1 is .The frequency of the reaction is approximately so the corresponding energy generation rate Psun is . This power agrees with measured values and others analytical sun power calculations results. Ninety percent of this power is produced within 23% of the radius where the temperature reaches 8-40 million Kelvin. Heat is transfer from the core to the surface through the so-called convective zone by radiation and convection mainly. At the surface called photosphere, the temperature drops to 5000 Kelvin. From the photosphere outward, there are two layers the chromospheres and the corona with a temperature gradient of 5000 Kelvin to 106 Kelvin. However, the density of these regions is very low ( . The reason for the high temperature of the corona is still unknown but its low density makes its contribution to solar radiation negligible. Sun power is radiated in all direction of the solar system. On the earth atmosphere upper layer, the radiation intensity or insolation is termed as the solar constant “ ”. By considering the surface enclosed by a radius equal to sun-earth distance and the sun power calculated previously, solar constant is approximated through the following formula to be or
26 (3.2)
where the solar constant, the sun radiated power, the sun radius.
The standard solar constant value has been subject to many debates from 1954 to 1983.
A value of is recognized by the world radiation center as the solar constant.
Figure 3.2. Sun-Earth Energy Transfer
3.1.3. Earth Surface Radiation
The extraterrestrial solar radiation spans over a width range of the electromagnetic spectrum. Particles having energy higher than the ultraviolet particles are almost entirely filtered out by the earth electromagnetic field. The solar constant spectral distribution is shown in table 1.1 It is an important information for solar application since each solar technology may only harvest energy contained in a specific
The extraterrestrial solar radiation spans over a width range of the electromagnetic spectrum. Particles having energy higher than the ultraviolet particles are almost entirely filtered out by the earth electromagnetic field. The solar constant spectral distribution is shown in table 1.1 It is an important information for solar application since each solar technology may only harvest energy contained in a specific